In a fluid pressure brake for a vehicle, an electric motor drives a pump for supplying a brake working fluid under pressure for operating the brakes. Such an electric motor must have strong power, while the same must be reduced in size and simple to assemble.
With reference to FIG. 1, a conventional electric motor is now described which has a thin cylindrical yoke 1 of a magnetic material having a closed end and an open end, and a plurality of permanent magnets 2 which are fixed to the inner peripheral surface of the yoke 1 at regular intervals in the circumferential direction. A rotor 3 is coaxially arranged within the yoke 1, and electromagnets 3a are arranged on the outer periphery of this rotor 3 along its circumference. A brush holder 5 for holding brushes 4, which are in sliding contact with commutators 3b for a rectifying function, is arranged on a side portion of the rotor 3 to cover the open end of the yoke 1. The outer periphery of the brush holder 5 is covered with a cover 6 which serves as an end surface cover for the yoke 1. Through-bolts 7, which are longer than the yoke 1 in the axial direction, are inserted to pass through bolt holes 6a provided in the cover 6 and notches 5a formed in the outer periphery of the brush holder 5. The inserted through-bolts 7 are received in the yoke 1 through clearances between adjacent ones of the permanent magnets 3 and bolt holes (not shown) formed in the closed end of the yoke 1. Thus, threaded portions provided on the forward ends of the through-bolts 7 are fitted with female screws or nuts which are provided on a fixed member (not shown), for fixing the electric motor to a fixed surface.
However, the conventional electric motor has the following problem: In order to drive a pump for a brake working fluid or the like, the electric motor must be reduced in size and increased in power. To this end, diametrical clearances between the permanent magnets 2 and the rotor 3 must be reduced to the minimum, and hence it is difficult to insert the through-bolts 7 through these small clearances. Therefore, the through-bolts 7 must be inserted, between the outer periphery of the rotor 3 and the inner periphery of the yoke 1, in the clearances between adjacent pairs of the permanent magnets 2. However, the through-bolts 7, which are made of magnetic material, are disadvantageously attracted by the permanent magnets 2 when the electric motor is mounted on a fixed surface (not shown) such as a braking system for a wheel or the like. This fact makes it difficult to assemble the known motor.